As the conventional technology, FIG. 26 shows the "Power supply unit for an electric discharge machine" disclosed in Japanese Patent Laid-Open Publication No. HEI 6-31534. In the figure, provided inside a machining liquid 2 filled in a machining liquid vessel 1 are a work 4 placed on a machining table 3 controlled by a numerical control apparatus or the like and an electrode 5 with the position thereof controlled by an electrode feeding means 6 for machining the work to a desired form, and a machining power is supplied from a power source 7 to a section between the electrode 5 and the work 4 through feeder lines 8A and 8B. Since the power source 7 is generally provided away from the electrode 5 as well a from the work 4, a length of each of the feeder lines 8A and 8B is in a range from 2 to 5 m. For this reason, these feeder lines 8A and 8B are wired close to each other or twisted for wiring to reduce inductance in the wiring. Accordingly, there is sometimes a case where capacitance between the feeder lines 8A and 8B increases.
FIG. 27 is a view showing the "Waveform control apparatus for an electric discharge machine" disclosed in Japanese Patent Laid-Open Publication No. HEI 7-68417. This unit is one example of the power source 7 for feeding a machining power to the electrode 5 and the work 4. The operations of the unit are disclosed in detail in the Invention described above, so that description thereof is omitted herein. When a switching element TR.sub.2 is turned ON, a machining power is supplied thereto, and when it is turned OFF, the power thereto is cut off. Although a diode 22 is not described in the Invention, there are many cases where the diode D.sub.22 is used because another power source is connected to the electrode 5 and the work 4. There are inductances 100, 101 of wiring in the feeder lines 8A, 8B respectively, and there is sometimes a case where the inductances 100, 101 of the wiring resonates to the static capacitance between the feeder lines 8A, 8B because of fluctuation of a voltage at the instant when electric discharge is generated between the electrode 5 and the work 4. FIG. 28 shows actual waveforms of a voltage 420 and a current 421 at the instant when electric discharge is generated therebetween. The voltage is a no-load voltage indicated at the reference numeral 424 and is around 87 V and a discharge current 421 is zero immediately before electric discharge is generated. When electric discharge is generated at the timing 423, the voltage abruptly drops to a discharge voltage indicated at 425 to be about 25 V. The discharge current 421 starts to flow and increase at the instant and on, and the current becomes constant at 30 A in this example. The above description indicates a phenomenon that inductance of the feeder line and the static capacitance resonate when the capacitance between the feeder lines is large, and electric discharge disappears when a current is negative or zero as a portion indicated by 426 in the figure. This phenomenon is called as split pulse, namely electric discharge is not appropriately generated, so that a machining speed may decrease or depletion of the electrode may increase.
In FIG. 27, when the switching element TR.sub.2 is turned OFF, a machining current becomes zero, and the diode D.sub.22 is tuned OFF, a junction capacitance of the diode D.sub.22 and the inductances 100, 101 of the wiring resonate to each other. FIG. 29 shows actual waveforms of an output voltage 430 and a current 421 from the power source 7 before and after the instant 433 when the switching element TR.sub.2 is turned OFF in a state in which electric discharge is being generated between the electrode 5 and the work 4. The voltage is a discharge voltage indicated by 425 and is about 25 V immediately before the switching element is turned OFF, and the discharge current 421 is 20 A in this example. When the switching element TR.sub.2 shown in FIG. 27 is turned OFF at the time indicated at 433, the voltage steeply drops to a voltage of around -60 V in a constant voltage body B.sub.20 indicated at 435. At this instant and on, the discharge current 421 is decreasing and becomes zero at the timing shown at 431. The diode D.sub.22 is turned OFF right after the current is zero, but a quite high-frequency voltage is generated as indicated at 432 because the junction capacitance and the inductances 100, 101 of the wiring resonate to each other. As for the discharge current, a resonance current flows between the electrode 5 and the work 4 as indicated at 434. This high-frequency voltage 432 gives bad influences over a control circuit as a noise, and the resonance current 434 in the discharge current causes increase of depletion of an electrode because the reverse current flows therein.
FIG. 30 shows a state of machining by the electric discharge machine equivalent to that shown in FIG. 26. The power source 7 and electrode 5, and the source 7 and the work 4 are connected to each other with the feeder lines 8A and 8B respectively. There is static capacitance between the lines because the feeder lines 8A, 8B are wired close to each other. If each of the lines has a length of several meters, the capacitance will be utmost several nanofarads. Also, static capacitance such as a junction capacitance in the switching element and that in the diode or the like exists in output from the power source because the power source is a semiconductor circuit. The capacitance therein is indicated by static capacitance C1. FIG. 31 shows a voltage and a current between the electrode 5 and the work 4 in the electric discharge machine shown in FIG. 30. The voltage 420 rises, and before electric discharge is generated, the static capacitance C1 is charged to no-load voltage at 424. Then, when electric discharge is generated at the timing indicated at 423, the voltage between the electrode 5 and work 4 becomes a discharge voltage 425. Accordingly, electric charge accumulated in the capacitance C1 flows as a large current to an electric discharge generating point A through the feeder lines 8A, 8B. This phenomenon is shown at the reference numeral 450 in FIG. 31, and if the inductance of the feeder lines is low, a pulse current 450 having the sharp leading edge as well as the peak at a high level flows for a short period of time. This pulse current 450 flows regardless of an amplitude of the discharge current 421. Accordingly, when the discharge current 421 has a small amplitude, namely when the discharge current is used for finishing electric discharge machining, the electrode 5 has, in many cases, a small size, which causes the electrode 5 to be substantially depleted due to this pulse current 450. If the feeder lines 8A, 8B are wired close to each other by twisting them or the like so that the inductance of the feeder lines will be as small as possible to enhance the performance of electric discharge machining, the static capacitance C1 increases and a peak value of the pulse current 450 becomes high, which causes depletion of the electrode to inconveniently increase, which is disadvantageous.
FIG. 32 is a view showing the "Power supply unit for an electric discharge machine" disclosed in Japanese Patent Laid-Open Publication No. HEI 6-31534. The power source 7, electrode 5 and work 4 are connected to each other with a coaxial cable 36. In a case where the wiring therebetween is executed through a coaxial cable as described above, inductance in the wiring is reduced with a high-speed response to a machining current improved, so that the performance of the electric discharge machine is improved. However, the capacitance 37 is large due to the characteristic of the coaxial cable and the inductance in the wiring is small, so that the surge current 450 shown in FIG. 31B becomes so high and the electrode is largely depleted, which causes the wiring through a coaxial cable to be infeasible.
FIG. 33 is a view showing an "Electric discharge machine for partial machining" disclosed in Japanese Patent Laid-Open Publication No. HEI 6-226538. A plurality of electrodes 5a, 5b, 5c and a work 4 are connected to the common power source 7. In a case where the connection therebetween is executed as described above, if electric discharge is generated in the electrode 5b, electrical charge accumulated in the static capacitance between the electrode 5a and the work 4 as well as in the static capacitance between the electrode 5c and the work 4 is flown into the electric discharge generating point A as indicated at 470. This current becomes larger as the number of electrodes provided therein increases. As described above, when electric discharge machining is executed by connecting the plurality of electrodes to the power supply unit, the surge current 450 shown in FIG. 31 becomes large, which causes the electrodes to be great depleted. As shown in FIG. 34, a plurality of power sources 7a, 7b, 7c may be connected to a plurality of electrodes 5a, 5b, 5c respectively, however, the method causes the cost to increase as well as the way of control to become complicated, so that it is found not to be practical.
FIG. 35 shows one example of an electric discharge detector for the electric discharge machine. In this detector, a voltage between an electrode 5 and a work 4 and a reference voltage 491 are inputted into a comparator 490, and an output 492 from this comparator 490 is outputted as a signal indicating detection of electric discharge.
FIGS. 36A to 36C show operations of this circuit. In FIG. 36A, when a machining voltage is applied thereto from the power source 7, the voltage between the electrode 5 and the work 4 is a no-load voltage 424. Then, when electric discharge is generated at the timing indicated at 423, the voltage becomes a discharge voltage 425 and a discharge current 421 flows therein as shown in FIG. 36B. If the reference voltage 491 is preset at the level indicated at 500, the signal 492 indicating detection of electric discharge is outputted as indicated at 501 when the voltage between the electrode 5 and the work 4 exceeds the reference voltage 500 as shown in FIG. 36C. However, there may frequently occur a phenomenon called immediate electric discharge that electric discharge is immediately generated before the voltage does not reach a no-load voltage to be a discharge voltage 425 as shown indicated by a dotted line at 501 in FIG. 36A. Accordingly, in the case as described above, the signal indicating detection of electric discharge is sometimes not outputted as shown at 502 in FIG. 36C. In this case, generation of electric discharge can not be detected, so that a period of time 503 while a discharge current 421 is flowing can not correctly be detected, which is disadvantageous.
FIG. 37 shows actual waveforms of the voltage 420 and the current 421 at the instant when electric discharge is generated between the electrode 5 and the work 4, which is the same as the case shown in FIG. 28. The voltage is a no-load voltage indicated at 424, around 87 V and a discharge current 421 is zero immediately before electric discharge is generated. When electric discharge is generated at the time indicated at 423, the voltage abruptly drops to a discharge voltage indicated at 425 to become around about 25 V. The discharge current 421 starts to flow at this instant and on, then increases, and the current becomes constant at 30 A in this example. This discharge current 421 very quickly increases, so that the current becomes around 13 A at the time indicated at 511 in 0.5 .mu.sec from generation of electric discharge and reaches around 27 A at the timing indicated at 512 in 1 .mu.sec therefrom. For this reason, even if a comparator with a high-speed response is used for the comparator 490 shown in FIG. 35 to obtain a signal 492 indicating detection of electric discharge according to the generation of electric discharge 423, the discharge current 421 has already risen, so that the timing 513 close to the rising edge of the discharging current 421 can not be controlled. Accordingly, there has not been such an electric discharge machine in which a discharge current can be controlled to arbitrary waveforms at the instant and on when electric discharge is started. For this reason, there has not been proposed any result of research on optimal waveforms with which depletion of an electrode is suppressed to a low level at the time of rising of a discharge current which causes most serious depletion of the electrode. It should be noted that a method called as a slope controlling, in which a discharge current is increased in a linear slope at the time when electric discharge is started and on, is used for products and it is understood from the method that the less the slope is inclined the less the electrode is depleted. However, if the slope has too small an inclination, a rising speed of a current is also slow after a point of time close to the starting time of the discharge current, so that an average value of a machining current drops, which causes a machining speed to become low.
FIG. 38 shows a view in which an X-axis in the "Slope-control waveform and characteristics thereof to a rising time" disclosed in Mitsubishi Denki Technological Report No. 6 Vol. 61 in 1987 is replaced with a changing rate of a current. This machining is executed under such conditions that an electrode is copper, a work is iron (SK3), a peak current is 11A, and a pulse width is 250 .mu.sec, and measurement is executed by changing a rising speed of a discharge current. It is understood from this figure that the electrode is less depleted when a changing rate (a rate of increase) of a current is lower. Also, the machining speed significantly drops at a lower portion of the changing rate of the current.
Also section of 2.1.2 "Machining with extremely low depletion in the cited reference includes the description" that "an entire current density is kept at a low level in accordance with expansion of an arc column, and as a result, the electronic current density is kept at a low level as described in the previous section so that depletion of the positive electrode can be reduced. If a slope controller is used, an electrode depletion rate of 0.1 to 0.01% can be obtained". However, in the slope controlling in which a discharge current is raised in a linear slope at the starting of the electric discharge and on, a current is increased at a constant speed from the low current up to the peak current, and such effects that the entire current density is kept at a constant level in accordance with expansion of the arc column (electric-discharge column) are effective up to the point of time immediately after start of the electric discharge, so that it is conceivable that increase of a current due to the slope may be slower as compared to increase of a cross-sectional area of the electric-discharge column after the current increases to a certain level, which is equivalent to a case where an average value of the current drops, and it is conceivable that the phenomenon causes a machining speed to significantly drop when the changing rate of the current is low.
A machining current in the conventional type of electric discharge machine is controlled as described above, so that there have been several problems such as that sometimes an electrode becomes very depleted as described above, that a machining speed is reduced, that electric discharge can not be detected in the instant of electric discharge, or that generation of electric discharge can not be detected before the discharge current begins.
It is an object of the present invention to obtain an electric discharge machine in which generation of electric discharge can be detected even in a state of the instant of electric discharge, generation of electric discharge can be detected before the discharge current starts up, depletion of an electrode can be reduced, and a machining speed does not drop.